The present invention relates to a fiber optic gyroscope including the function to serve as a polarizer and a substrate-based optical integrated circuit on which a branching optical waveguide is formed for detecting an angular rate about a coil axis which is applied to a fiber optic coil.
In the fiber optic gyroscope, light beams impinge on opposite ends of the fiber optic coil, and propagate therethrough to be emitted from the opposite ends. A phase difference between the both emitted light beams remains to be zero when there is no angular rate which is applied to the fiber optic coil about the axis thereof. However, upon application of an angular rate, the phase difference between the both emitted beams changes in accordance with the angular rate applied. Hence, the angular rate applied to the fiber optic coil is detected by detecting the phase difference. When the fiber optic coil exhibits birefringence, a light beam which propagates through the coil has a propagation velocity for one and another propagation velocity for the other of two mutually orthogonal linearly polarized components. Accordingly, similar linearly polarized beams each representing one of linear polarizations, which are emitted after propagation through the fiber optic coil, are led into interference with each other in order to detect the angular rate. To obtain an interference beam between the beams having the selected linear polarization, a polarizer is used. The fiber optic coil is formed by an optical fiber that maintains the plane of polarization so that a linear polarization that is chosen by the polarizer can propagate through the fiber optic coil.
However, an optical fiber which maintains the plane of polarization is much more expensive than a single mode optical fiber. In a single mode fiber optic coil in which a single mode optical fiber is used as a fiber optic coil in consideration of such expensiveness, birefringence occurs, though slightly, when the coil is flexed, leading to the consequence that the linear polarization of the beam propagating through the coil is greatly susceptible to some influence to change. Current practice uses depolarizers to produce one and the other linear polarization of an equal amplitude or to achieve non-polarization before the beams impinge on the opposite ends of the fiber optic coil.
There is a proposal for a substrate-based optical integrated circuit having an optical waveguide which has the function to serve as a branching filter which produces an interference between light beams emitted from the opposite ends of a fiber optic coil after light from a light source is branched to impinge on the opposite ends of the fiber optic coil and as a polarizer which produces a given linear polarization. When this substrate-based optical integrated circuit is used with a single mode optical fiber to form a fiber optic gyroscope, a light beam which impinges on the optical waveguide on the substrate-based optical integrated circuit from the light source propagates in TE mode, which represents a propagation mode of the optical waveguide, and is branched into two beams to impinge on the opposite ends of the fiber optic coil while a light beam in TM mode, representing an extinction mode of the optical waveguide, leaks out of the optical waveguide, and is reflected by the bottom surface of the substrate of the substrate-based optical integrated circuit to impinge on the fiber optic coil. In other words, the extinction ratio of the polarizer is degraded as a result of the incidence of not only the linear polarization which is based on the TE mode beam, but also of the linear polarization which is orthogonal thereto, which occurs as the incidence of a stray beam from the leaking TM mode beam. To solve this problem, U.S. Pat. No. 5,475,772, issued Dec. 12, 1995, proposes forming a space filter in the bottom surface of the substrate of the optical integrated circuit. However, it is found subsequently that the leaking TM mode beam not only impinges on the fiber optic coil as the TM mode beam, but is also converted into the TE mode before it impinges on the fiber optic coil, thus producing an adverse influence upon the original TE mode beam which has propagated through the optical waveguide to degrade the extinction ratio of the polarizer.
A fiber optic gyroscope which overcomes these difficulties associated with the use of the substrate-based optical waveguide is described in the Proceedings of SPIE, Vol. 2292, pp. 166–176. This fiber optic gyroscope will now be described with reference to FIG. 1.
A light beam which is emitted from a light source 10 propagates through a first optical fiber 11, an optical fiber coupler 12 and a second optical fiber 13 to impinge on a substrate-based optical integrated circuit 14. The first optical fiber 11, the optical fiber coupler 12 and the second optical fiber 13 are all formed by optical fibers which maintain the plane of polarization. The optical fiber coupler 12 comprises a pair of optical fibers which maintain the plane of polarization and which are fused together intermediate their length and thus the cores of the both fibers are disposed close to each other. The optical integrated circuit 14 comprises a substrate of optical crystal of lithium niobate (LiNbO3) on which a Y-branch optical waveguide 15 is fabricated by the proton exchange method and light modulators 16 and 17 are formed in the region of the respective branches of the optical waveguide.
The light beam which is incident on the Y-branch optical waveguide 15 is branched into a first and a second light beam. The first light beam is incident on one end of a fiber optic coil 20 through a third optical fiber 18, and then propagates through the coil 20 in the clockwise direction (which is referred to hereafter as CW direction). The second branched light beam is incident on the other end of the fiber optic coil 20 through a fourth optical fiber 21 and propagates therethrough in the counter-clockwise direction (here after referred to as CCW direction). The third and the fourth optical fiber 18 and 21 are constructed by optical fibers which maintain the plane of polarization. It should be understood that these optical fibers 18 and 21 which maintain the plane of the polarization are connected to the optical waveguide 15 of the optical integrated circuit 14 such that their inherent axes (generally, the direction of the electric field of the linear polarization having a higher propagation velocity) form an angle of 45° with respect to the direction of the electric field of the propagating TE mode in the optical waveguide 15, thus functioning as a depolarizer together with the optical waveguide 15. The fiber optic coil 20 is constructed with a single mode optical fiber.
As the fiber optic coil 20 rotates about an axis thereof, there occurs a phase difference between the light beam propagating in the CW direction and the light beam propagating in the CCW direction through the fiber optic coil 20, and when these light beams impinge on the Y-branch optical waveguide to be coupled together, an interference beam is produced. The interference beam is branched by the optical fiber coupler 12 to be incident on a light receiver 25, which in turn delivers an electrical signal which depends on the intensity of the interference beam. The output signal from the light receiver 25 is supplied to a detection circuit 26.
The light modulator 17 is used in order to enhance a detection sensitivity. A phase modulation signal (which may be a sinusoidal signal, for example) is applied to the light modulator 17 from a modulation signal generator 27, thus phase modulating the light beam which is branched to propagate through one of optical waveguides. A signal which is synchronized with the phase modulation signal is supplied from the modulation signal generator 27 to the detection circuit 26 to enable a synchronous detection of the electrical signal delivered from the light receiver 25.
A detection output from the detection circuit 26 which depends on the input angular rate is supplied to a feedback signal generator 28, which then generates a feedback signal which depends on the magnitude of the detection output which is input thereto. This feedback signal is supplied to the light modulator 16 to allow a control to be exercised so that the detection output from the detection circuit 26 becomes equal to zero. An output signal from the fiber optic gyroscope is derived from the feedback signal generated by the feedback signal generator 28. In the arrangement shown in FIG. 1, every optical path disposed between the light source 10 and the optical integrated circuit 14 and extending through the optical fiber coupler 12 is formed by an optical fiber which maintains the plane of polarization. An optical fiber which is designed to maintain the plane of polarization utilizes its birefringence to avoid a change in the plane of polarization during the propagation of a light beam. However, because of the birefringence, there is produced a difference in a propagation velocity between the two orthogonal linear polarizations which propagate through the optical fiber. The optical path located between the light source 10 and the optical integrated circuit 14 requires a length on the order of 1 m, and accordingly, as for the coherence of a light beam which is emitted from a light source such as a super-luminescence diode (SLD) as used in a usual fiber optic gyroscope, the coherence between the both linear polarizations is eliminated if the light beam emitted from the light source propagates through the polarization maintaining optical fiber through a distance on the order of ten and several centimeters inasmuch as a distance between the wave fronts of the both linear polarizations which results from the differential propagation velocities of the both linear polarizations is well in excess of the coherence. Consequently, one of the linear polarizations from the polarization maintaining optical fiber 13 propagates thorough the optical waveguide in the TE mode which represents the propagation mode in the optical waveguide of the optical integrated circuit 14, and if the other linear polarization is incident on the optical waveguide in its TM mode to leak out of the optical waveguide to produce a stray beam in the TM mode or is converted into the TE mode to be recoupled irregulary to the optical waveguide or the polarization maintaining optical fibers 18 or 21, the recoupling causes no influence whatsoever on the detection of the angular rate because there is no coherence between the linear polarization which propagates through the optical waveguide and the stray beam. As a consequence, the need to form a space filter in the bottom surface of the substrate of the optical integrated circuit 14 in order to suppress the influence of the stray beam is avoided.
In the fiber optic gyroscope shown in FIG. 1, the polarization maintaining optical fibers 18 and 21 have a length L which is chosen to remove the coherence between the two orthogonal linear polarizations which propagate therethrough to a satisfactory degree. The length is chosen to satisfy the following requirement:Lλ/B>Lc  (1)where B represents a beat length (typically 2 mm), λ the wavelength of the light beam (typically 0.83 μm), and Lc the coherence length of the beam (typically 50 μm for a fiber optic gyroscope). When these typical values are substituted into the above inequality, we have L>0.12 m. In the above inequality, L/B represent a phase difference occurring due to differential propagation velocities of the orthogonal linear polarizations, and Lλ/B a distance between wave fronts of the orthogonal linear polarizations.
The both polarization maintaining optical fibers 18 and 21 are connected to the optical waveguides so that their polarization axis form an angle of 45° with respect to the direction of the electric field of the TE mode in the optical waveguide of the optical integrated circuit 14. Accordingly, the combination of the optical waveguide and either polarization maintaining optical fiber 18 or 21 forms a depolarizer, and light beams from the optical integrated circuit 14 impinge on the opposite ends of the single mode fiber optic coil 20 in a non-polarized condition.
When the described arrangement is used, if the fiber optic coil 20 is constructed with a single mode optical fiber, the angular rate can be detected with a good accuracy. Above described difficulties involved with the use of a single mode fiber optic coil and the use of a substrate-based optical integrated circuit which has a combined function serving as a polarizer and a branching filter appear as a variation in the bias error of the fiber optic gyroscope. Specifically, when no angular rate is applied, the fiber optic gyroscope should deliver a detection output of zero, but there is a detection output as a bias, and the bias error varies, thus degrading the accuracy of detection.
Literatures quoted above do not mention about the length of the both polarization maintaining optical fibers 18 and 21, but there is a description about such length in the Proceedings of SPIE, Vol. 2070, pp. 152–163. Specifically, the lengths of the both polarization maintaining optical fibers 18 and 21 are chosen in the ratio of 1:2 so that they can function as LYOT-type depolarizer, and the difference between the both lengths is chosen to provide a phase difference which avoids a mutual interference between the both linear polarizations for the worst phase conditions for the linear polarizations caused by the refringence of the single mode fiber optic coil 20.
The connection of the polarization maintaining optical fiber to the end face of the optical wave guide 15 of the substrate-based optical integrated circuit 14 takes place by mounting a carrier formed of the same material as the substrate of the optical integrated circuit 14 on the end of the polarization maintaining optical fiber to be adhesively secured, followed by adhesively securing the end face of the carrier to the end face of the substrate-based optical integrated circuit 14. More specifically, as illustrated in FIG. 2, a carrier 31 comprises a square pillar, one side of which is formed with a fiber retaining groove 32, into which one end of the polarization maintaining optical fiber 13 is inserted. While maintaining the end face of the optical fiber 13 in alignment with one end face of the carrier 31, and while visually recognizing the end faces of two stress applicators 13a which are disposed within the optical fiber 13a on the opposite sides of the axis and extending parallel to the axis on a microscope, the optical fiber 13- and the carrier 31 are secured together using an adhesive 34 so that a direction 33 in which the two stress applicators 13a are arrayed remains parallel to one lateral surface (reference surface) of the carrier 31. In this instance, when the hair line of the microscope is maintained in coincidence with the reference surface 31a of the carrier 31, the direction of array 33 can be made parallel to the reference surface 31 in a facilitated manner and with good accuracy.
Subsequently, the bottom surface of the substrate of the optical integrated circuit 14 is mounted on a connector in alignment with a reference surface thereof, and the carrier 31 is mounted on the connector so that a lateral surface 31b which is perpendicular to the reference surface 31a of the carrier 31 is in alignment with the reference surface of the connector. The connector is then operated to controllably move the carrier 31 in the vertical and the horizontal direction with respect to the reference surface of the connector so that the optical waveguide 15 and the optical fiber 13 are located in alignment with each other as shown in FIGS. 3A and 3B, and the end faces of the carrier 31 and the optical fiber 33 are brought into contact with the end face of the optical integrated circuit 14, which are then adhesively secured together. In this manner, the polarization axis of the polarization maintaining optical fiber 13 coincides with the direction of the electric field of the TE mode of the optical waveguide 15. Normally, the direction which is perpendicular to the direction of array 33 of the two stress applicators 13a (rapid phase axis) is chosen to be coincident with the direction of the electric field of the TE mode. This case is shown in FIGS. 3A and 3B.
When connecting the polarization maintaining optical fiber 18 or 21 to the optical waveguide 15, the carrier 31 is initially mounted on the polarization maintaining optical fiber 18. In this instance, the polarization maintaining optical fiber 18 and the carrier 31 are adhesively secured together after visually recognizing on a microscope that the direction of array of the two stress applicators 18a are disposed at an angle of 45° with respect to the reference surface 31a, as shown in phantom lines in FIG. 2. The carrier 31 is subsequently secured to the optical integrated circuit 14 in the similar manner as mentioned above with reference to FIG. 3. The connection between the polarization maintaining optical fiber 21 and the optical waveguide 15 takes place in the similar manner.
However, making the direction in which the stress applicators are arrayed to be disposed at an angle of 45° with respect to the reference surface 31a by a visual recognition can only be achieved with a poorer accuracy of angular alignment as compared with making the array direction of the stress applicators to be parallel to the reference surface 31a. By way of example, an offset of the angular alignment when making the direction of the array to be parallel to the reference surface 31a is several degrees per hour as considered in terms of the bias error of the fiber optic gyroscope, but a corresponding offset in the angular alignment when making the direction of array to be at an angle of 45° with respect to the reference surface 31a increases to as much as several tens of degrees per hour as considered in terms of the bias error. This means that if the polarization axis of each of the polarization maintaining optical fiber 18 or 21 is not disposed properly at an angle of 45° with respect to the direction of the electric field of the TE mode of the optical wave guide 15, a light beam which is incident on the single mode fiber optic coil 20 cannot be in an exactly non-polarized condition, leading to a degradation in the accuracy of detecting the angular rate.
An open loop fiber optic gyroscope which uses a substrate-based optical integrated circuit is disclosed in Japanese Patent Kokai Publication No. 29,184/96 (published Feb. 2, 1996), for example. The difficulties mentioned above which occur in the substrate-based optical integrated circuit can similarly be overcome for such an open loop fiber optic gyroscope when every optical path of the optical integrated circuit which is disposed toward the light source is constructed with a polarization maintaining optical fiber as shown in FIG. 1 and when a polarization maintaining optical fiber is inserted between the optical integrated circuit and the fiber optic coil. However, the issue of the angular alignment when connecting the optical integrated circuit and the polarization maintaining optical fiber which is disposed toward the coil remains in the similar manner.